1. Field of the Invention
This invention relates to a pH sensor, particularly, to a pH sensor of the so-called polymer film-coated electrode type.
2. Description of the Prior Art
A pH sensor of the type wherein the surface of an electrode formed of an electrically conductive material is coated with a hydrogen ion-sensitive polymer film is under development. Said pH sensor mentioned is used for measuring, for example, the hydrogen ion concentration of an aqueous solution by utilizing a potential response. A known pH sensor of this type comprises an electrode formed of a noble metal, particularly, platinum and coated with a polymer film, e.g., a film of polymer of phenol, diaminobenzene or the like. This pH sensor can be miniaturized, exhibits a rapid response time and, thus, is expected to be capable of insertion into a living body for measuring the pH of a body fluid. However, where the solution to be measured contains additional components such as micro ions, organic acids, basic compounds, medium molecular weight compounds (e.g., proteins and cleatinin) and nucleic acids (e.g., purines) as well as hydrogen ions, the pH sensor mentioned above tends to be affected by the additional components. Also, where dissolved oxygen is present in the solution to be measured, the potential value tends to fluctuate under the influence of the dissolved oxygen. The difficulty is thought to be derived from the use of a noble metal such as platinum, which is highly sensitive to oxygen, as the electrode substrate.